1. Field of the Invention
The invention concerns an integrated electro-optical modulator/commutator with pass-band response.
When an electro-optical modulator is made, generally, at high frequencies, it can no longer be considered to be a component with localized constants but a component with distributed constants because its dimensions are in the order of the wavelength of the applied modulating electrical wave. This leads to making electrodes in the form of electrical transmission lines for which the characteristic impedance is chosen as a function of the use. Owing to the difference in frequencies between guided optical waves and modulation electrical waves, these two waves are propagated with different propagation constants (effective refraction indices) resulting in a limitation in the pass-band of the modulators.
Thus, for a phase modulator as shown in FIG. 1, comprising an optical guide G subjected to an alternating electrical field produced by two electrodes E1 and E2 framing the guide G and a current source S, it can be shown that the phase shift induced in a device such as the one shown in FIG. 1 is strictly null at the frequency: ##EQU1##
with
L, the interaction length,
c, the speed of light in a vacuum,
n.sub.e, the effective electrical index,
n.sub.o, the effective optical index.
The physical explanation of this phenomenon is that, at this frequency, a group of photons flowing through the modulator experience a positive alternation of the modulation field in the first half of the device and a negative modulation in the second half. For electro-optical modulators by Pockel's effect, the variation in index is a linear function of the field applied and, hence, the total phase shift is null. This phenomenon can be compared with the so-called "phase matching" problem in linear optics.
The pass-band of a device of this type is therefore directly related to the properties of the substrate at optical and electrical frequencies as well as to the interaction length.
In certain applications, it is worthwhile to use modulators for which the frequency response is not of the low-pass type (null frequency to maximum frequency) but rather of the band-pass type: f.sub.1 .rarw..fwdarw.f.sub.2 around an operating frequency f.sub.o (the transmission of radar signals for example).
For the phase modulator described above, it is then possible to use the configuration of electrodes shown in FIG. 2. According to this configuration, there are provided two pairs of electrodes E3-E4 and E5-E6 supplied with AC power by a voltage source S. The connection of the electrodes is intersected so that the electrode E3 is connected to the electrode E6 and the electrode E4 is connected to the electrode E5. This configuration enables the reversal of the signal of the field applied in the two halves of the modulator. In this case, at null frequency, the accumulated phase shift is exactly null. On the contrary, at the frequency given by the preceding equation, the phase shift is at its maximum. It may be noted that, through the symmetry around the frequency f.sub.o, the pass-band of the device of FIG. 2 is twice that of the device of FIG. 1 for the same total length of electrodes.
Naturally, this simple principle can be applied to multiple section electrodes to increase the operating frequency if the interaction length is fixed.
When an amplitude modulator is preferable, this principle can be applied to one or both branches of an interferometer, the basic drawing of which is shown in FIG. 3. This interferometer has two guides G1 and G2, one input guide E1 and one output guide S1.
In the examples given above, to work around the frequency f.sub.o, it becomes necessary to split the control electrodes into several parts or at least to use configurations, the complicated nature of which makes them difficult to use at high frequencies.
The invention relates to a device enabling the use of a standard electrode, with the resonance around f.sub.o being obtained optically.
The basic principle of the device according to the invention is that it makes use of this difference between optical propagation and electrical propagation constants rather than trying to compensate for this difference by multiple-section electrodes.